Heat-recoverable articles, such as heat-shrinkable tubes and heat-shrinkable caps, are used as covering for joints between insulated electrical wires, wire terminals, and metal pipes for the purpose of waterproofing, corrosion protection, and the like. For example, when a joint between insulated electrical wires is covered with a heat-shrinkable tube and heating is performed, the heat-shrinkable tube shrinks, conforms to the shape of the joint, and adheres thereto, and thus the joint can be protected.
Examples of such heat-recoverable articles include a multilayered heat-recoverable article in which an adhesive layer is provided on the inner surface of a heat-shrinkable base material layer (refer to Japanese Unexamined Patent Application Publication No. 2000-129042). As the adhesive layer, a hot-melt adhesive, such as an ethylene-vinyl acetate copolymer (EVA), an ethylene-ethyl acrylate copolymer (EEA), or a polyamide resin, is used. Furthermore, the adhesive layer may be formed so as to include two thermoplastic resin layers (refer to Japanese Unexamined Patent Application Publication No. 8-230037 and Japanese Patent No. 4019524).
On the other hand, a multilayered heat-recoverable article is produced by a method in which a heat-shrinkable base material layer and an adhesive layer are each extruded into a tubular shape, then the resulting tubular body is expanded in the radial direction (the diameter is expanded) under heating, and cooling is performed to fix the shape. In some cases, in order to improve heat resistance, the extruded multilayered heat-recoverable article may be irradiated with ionizing radiation so that the heat-shrinkable base material layer can be crosslinked.
When the multilayered heat-recoverable article is used, by performing heating with an adherend being covered with the multilayered heat-recoverable article, heat shrinkage of the heat-shrinkable base material layer occurs, and at the same time, the adhesive layer is fluidized. At this time, the space between the adherend and the heat-shrinkable base material layer is filled with the fluidized adhesive layer, and thereby, the multilayered heat-recoverable article is made to adhere closely to the adherend.
One example of the adherend is a PVC insulated wire whose outermost layer is insulated by a polyvinyl chloride (PVC) layer. In the case where the multilayered heat-recoverable article is made to adhere to a PVC insulated wire, there is a concern that the polyvinyl chloride layer may be deteriorated depending on the type of multilayered heat-recoverable article, for example, depending on the composition of the adhesive layer.
Furthermore, flowability of the adhesive layer is increased by heat generated when current is passed through the PVC insulated wire, and therefore, it is assumed that the adhesive layer flows out from the heat-shrinkable base material layer. In the case where the adhesive layer flows out, adhesion between the multilayered heat-recoverable article and the PVC insulated wire is lost, and as a result, it is not possible for the multilayered heat-recoverable article to protect the PVC insulated wire from external force, moisture, and the like.